How Precipitation FormsForms of PrecipitationPrecipitation MeasurementsIntentional Weather ModificationInadvertent Weather Modification3/04/03Outline for Lecture 0 standard atmosphere 300mb 30,000 ft standard atmosphere 500mb 18,000 ft standard atmosphere 700mb 10,000 ft standard atmosphere 850mb 5,000 ft How Precipitation FormsImportant Facts!"cloud droplets are tin#$%0 micrometers&'"man# condensation nuclei are present' "tin# particles fall moreslowl# than lar(e ones'"a cloud droplet)s diameter must (row*%00 times to reach a raindrop)s diameter'"to attain the volume ofa rain droplet+ the clouddroplet increases a million times in volume' ,he -er(eron Process $.old .louds& ,he .ollision".oalescence Process $Warm .louds&Precipitation Formation Mechanisms ,he -er(eron process descri/es how rain or snow forms when the cloud temperature is /elow free0in('This process where ice crystals grow at the expense of cloud droplets is called the Ice .r#stal Process. It is also named after the Norwegian researcher who discovered it (,or -er(eron+ there were others&. TemperatureRH wrt* H

!(li"# RH wrt H

!(ice# $%& '$$( '$$()$*%& '$$( '$*()'$%& '$$( ''$()'*%& '$$( ''*()$%& '$$( ''(1wrt + with respect to ,hree important properties of water droplets!'.loud droplets do not free0e at 0'2.%' 3upercooled $water in the liquid state below 0C& water droplets will free0e immediatel# if a(itated sufficientl# or when the# come in contact with free0in( nuclei $a cr#stalline structure similar to ice&3',he saturation vapor pressure with respect to ice is lower than the saturation vapor pressure with respect to li4uid water' Ice3aturation with 5espect to Ice and Water6apor pressure is the pressure due to water vapor molecules when the evaporation rate is e4ual to the condensation rate'-ecause of the cr#stalline structure of ice+ water molecules are not a/le to /rea7 free from anice surface as easil#thanfrom a water surface' ,herefore+ the saturation vapor pressure with respect to an ice surface would /e less than the saturation vapor pressure with respect to a li4uid water surface at a (iven temperature'Li4uid Water 8rowth Process /# -er(eron Process.oe9istence of supercooled water droplets and ice cr#stals is essential to precipitation process in the Mid latitudes'"02."02."402. %0+000 ft3+000 ft+000 ftWater :roplets3upercooled water droplets and ice cr#stalsIce cr#stals topmiddlelower"%02. ;+000 ft 8ravitational Force< Frictional Force,erminal 6elocit# occurs when! F < 0$when F(ravit# < Ffriction & = 4/3 = r3 =(< = r%= v = 4/3 = ( = r mm 0'> @ > mm?ppro9imate si0e of t#pes of Precipitation Forms of Precipitation(Rain, Snow, Sleet and Glaze, Hail):roplet si0e determines the t#pe of precipitation' 5ain is the term for drops of water that fall from a cloudand have a diameter of 0'> millimeter $mm&':ri00le and misthave smaller droplets'5ain mostl# occurs in nim/ostratus clouds and cumulonim/usclouds',hese clouds are capa/le ofproducin( cloudbursts.Most rain starts as snow or ice cr#stalsA as the snow falls throu(hthe cloud it melts':ri00le is a fine uniform water droplet with adiameter less than 0'> mm' Rain ,emperature Profile for 5ain 3now is precipitation in the form of ice cr#stals $snowfla7es& or more often+ a((re(ates if ice cr#stals',he si0e and structure of the cr#stals is a function of the temperature at which the# form'When air temperatures are cold the moisture content is ver#small',his results in the formation of ver# li(ht fluff# snowmade up of si9 sided ice cr#stals'When conditions are warmer+ the ice cr#stals Boin to(ether intolar(er clumps consistin( interloc7ed a((re(ates of cr#stals'Snow ,emperature Profile for 3now Sleet and Glaze3leet is a wintertime phenomenon that refers to the fall of small particles of ice that are clear to translucent'3leet forms when rainpasses throu(h a coldla#er of air and free0esinto ice pellets',hisoccurs most often in the winter when warmair is forced over a la#erof cold air' ,emperature Profile for 3leet and 8la0e HailHail is precipitation in the form of hard+ rounded pellets orirre(ular lumps of ice',he la#ers of ice accumulate as the hailstone travels up and down in a stron( convective cloud'Hailstones /e(in as smallice pellets that (row /# addin(supercooled water droplets asthe# move throu(h the cloud'?sthe ice cr#stal c#cles up and downin the cloud the hailstones increasein si0e until the# are forced out /#a downdraft or /ecome heav#enou(h to fall out' +"is lar,e "ailstone fell from a tornadic supercell nort"east of %recenrid,e- +.( +"e stones left a pat" miles /ide littered /it" t"ree to four and one "alf inc" stones( +"e "ail fell from a dar cloud 0ase to t"e sout" of a de'elopin, mesocyclone( 1o rain or t"under occurred in t"e immediate area /"ile t"e stones /ere comin, do/n(Hail4C Rime5ime is a deposit of ice cr#stals formed /# the free0in( of super cooled fo( or cloud droplets on o/Bects whose surface temperature is /elow free0in('When rime forms on trees+ it covers them with ice feathersA in wind# conditions onl# the windward surfaces will accumulate the la#er of rime' ,a/le >"4 Precipitation Measurements3tandard Instruments!3imple rain (au(e+ standard rain (au(e+ tippin( /uc7et (au(e+ wei(hin( (au(e 5adar 3ummar#8ood estimates of rainfall amounts can /e o/tain from radar summaries'5adar is less relia/le for fro0en precipitation'.overa(e still sparse in man# locations' Tropical Rain easurin! ission (TR)Ideali0ed 8lo/al Precipitation 5epresentation ?s of late DD;+ measurements of the (lo/al distri/ution of rainfall at the EarthFs surfacehad uncertainties of the order of >0G and the(lo/al distri/ution of vertical profiles of precipitation was far less well determined' ,5MM will provide the firstspace/orne rain radar andmicrowave radiometric datathat will measure the verticaldistri/ution of precipitationover the tropics in a /and /etween H 3>F in latitude' 3uch information will (reatl#enhance our understandin(of the interactions /etweenthe sea+ air and land masses which produce chan(es in (lo/al rainfall and climate',5MM o/servations will alsohelp improve modelin( of tropical rainfall processes andtheir influence on (lo/al circulation leadin( to /etterpredictions ofrainfall and itsvaria/ilit# at various time scales' ,ropical 5ainfall Measurin( Mission (TR) Intentional Weather Modification,he deli/erate human intervention to influence atmospheric processes that constitute the weather'Much of the motivation to attempt weather modification techni4ueswas /ased on anecdotal evidence'5esultswere never conclusive'Hail cannons where popular in the lateI00)s+ with the /elief that inBection of smo7eparticles into developin( clouds could increase the ice nuclei and reduce the si0eof hailstones or suppress hail alto(ether' .harles M' Hatfield $"#$%&"'(#&Lots of people in 3outhern .alifornia 7new a/out Hatfield+ who /# then rated fol7 hero status' He called himself the Moisture ?ccelerator+ /ut others 7new him /est as ,he 5ainma7er' 3ince D0% when his first e9periments with chemicals and Jevaporatin( tan7sJ dampened his fatherFs ranch near Oceanside+ Hatfield had pleased people in Los ?n(eles and farmers in the .entral and 3an Koa4uin valle#s /# fulfillin( hundreds of their rainma7in( contracts' The oisture )ccelerator* D4L 6incent K' 3chafer discovered that dr# ice dropped into supercooled clouds spurred the (rowth of ice cr#stals whicheither induced precipitation or dispersed fo( or clouds'.loud 3eedin( In order for cloud seedin( to tri((er precipitation+ conditions must/e Bust ri(ht'.louds must /e presentA seedin( cannot create clouds'? portion of the clouds must contain supercooled water'One method assumes that the clouds are lac7in( in free0in( nucleiand addin( them will stimulate precipitation /# the -er(eron process' One must /e careful not to overseed as this will produce too man#+ too small ice cr#stals' .loud 3eedin( Precipitation is a complicatedprocess' ?fter fift# #ears of cloud seedin( e9perimentation and stud#the scientific communit# is stilllac7in( a full understandin( of precipitation processes' .loud 3eedin( Frost Prevention,emperature 0onation durin(an inversion' ,emperature increases with hei(ht to the topof the inversion and then decreases'Frost protectiontechni4ues use the warmer air a/ove the crop asa heat source' http,--www.atmos.umd.edu-.meto$$-meto$$exam'review.htm